To a mathematician, a tree is a data structure, starting with the root or node at the top, branching out and terminating in leaves at the bottom.
To an ecologist, a tree is a unit capable of casting shade on other plants and thus competing with them for life-giving sunlight.
To a forester, trees are woody plants having an erect stem at least 7.5 cm in diameters and 4 m in height, with a crown of leaves.
To a botanist, a tree is rigid, so only those plants that produce wood or lignin–a kind of tissue–are trees.
And to the rest of us free-rangers, yes, a tree is a living being, but it can’t move.
*I’m paraphrasing Francis Hallé.